72 ON THE PHYSIOLOGICAL CAUSES OF 



Lighter and more mobile elastic bodies, as for example 

 strings, can be set in motion by a much smaller number of 

 aerial impulses. Hence they can be set in sympathetic motion 

 much more easily than tuning-forks, and by means of a musical 

 tone which is far less accurately in unison with themselves. 



Now, then, if several tones are sounded in the neighbour- 

 hood of a pianoforte, no string can be set in sympathetic 

 vibration unless it is in unison with one of those tones. For 

 example, depress the forte pedal (thus raising the dampers), and 

 put paper riders on all the strings. They will of course leap 

 FIG. 7. 



off when their strings are put in vibration. Then let several 

 voices or instruments sound tones in the neighbourhood. All 

 those riders, and only those, will leap off which are placed upon 

 strings that correspond to tones of the same pitch as those 

 sounded. You perceive that a pianoforte is also capable of 

 analysing the wave confusion of the air into its elementary con- 

 stituents. 



The process which actually goes on in our ear is probably 

 very like that just described. Deep in the petrous bone out of 

 which the internal ear is hollowed lies a peculiar organ, the 

 cochlea or snail shell a cavity filled with water, and so called 



